The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and an image forming method and more particularly to an image forming apparatus of the type causing colored particles to fly for forming an image on a paper sheet or similar recording medium.
An electrophotographic process has been extensively applied to a copier, printer, facsimile apparatus or similar image forming apparatus. Typical of the electrophotographic process is a Carlson method (xerography). However, the problem with the Carlson method is that it needs a charging step, an exposing step, a developing step, an image transferring step, a fixing step and a cleaning step, i.e., six consecutive steps in total. Such a process is not practicable without resorting to a sophisticated, bulky construction. Japanese Patent 2,897,705 discloses a simple electrophotographic process that is a substitute for the Carlson method. The electrophotographic process taught in this document does not charge a photoconductive element and thereby reduces the number of steps (Prior Art 1 hereinafter).
Japanese Patent No. 1,876,764 teaches an electrophotographic recording method directed toward a higher toner transfer speed and the obviation of fog (Prior Art 2 hereinafter). Prior Art 2 includes a toner carrying member made up of a transparent base, a transparent electrode, and a carrier transport layer. Toner formed of a carrier generating material is charged by friction and caused to deposit on the surface of the toner carrying member. Light selectively scans the toner via the transparent base of the toner carrying member in order to invert the polarity of the toner. A transfer electrode is positioned behind a paper sheet or similar recording medium and biased to negative polarity. The transfer electrode causes the toner inverted in polarity to electrostatically move toward the paper sheet.
Further, Japanese Patent Laid-Open Publication No. 7-253704 proposes an image forming apparatus constructed to obviate defective image transfer, e.g., the adhesion of toner and fog (Prior Art 3 hereinafter). In Prior Art 3, photoconductive toner is charged to negative polarity by friction and coated on a transparent, conductive carrying member. When the toner is exposed, the resistance of the toner lowers with the result that the negative charge of the toner flows to the above carrying member. A power supply forms an electric field for image transfer between the carrying member and a facing electrode facing the carrying member via a gap. The power supply injects positive charge in the toner by contact induction charging. As a result, the toner flies toward the facing electrode via the gap and deposits on a recording medium.
Prior Art 1, however, gives rise to some problems that will be described specifically later.
As for Prior Art 2, when an organic carrier generating material is used, light causes electron-hole pairs to be generated in the material. Prior art 2, however, does not address to a problem that a high-tension electric field is essential for electrons and holes to separate from each other and migrate at a practical speed. Specifically, a practical electric field does not cause the particles to fly or needs a long period of time for the migration of charge and the flight of the particles, failing to implement a practical printing speed. More specifically, it is known that an electric field as high as 104 V/cm is necessary for electrons and holes in an organic material to separate from each other or for a separated charge carrier to migrate at a sufficiently high speed. Such a value is of the order of a breakdown start electric field of air. Should the high-tension electric field be applied between transferring means and a transparent electrode included in Prior Art 2, the breakdown of air would occur. That is, Prior Art 2 cannot exceed the above value of the electric field and therefore cannot solve the above practicality problem.
Prior Art 3 teaches that when photoconductive toner is exposed under a preselected electric field for transfer, the resistance of the toner lowers with the result that charge is injected from an electrode into the toner. Generally, however, the resistance of toner and therefore an electric field that causes the toner to start flying on the basis of charge injection is irregular. Prior Art 3 relies only on an electric field for image transfer and therefore sometimes causes even the toner in unexposed portions to start flying, resulting in a fog image.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Publication No. 5-88837.
It is therefore an object of the present invention to provide an image forming apparatus capable of forming a high-resolution, fog-free image, using even an organic photoconductive material, and realizing a simple, highly practical process for causing toner to fly toward a recording medium.
In accordance with the present invention, an image forming apparatus for causing photoconductive, colored particles to deposit on a recording medium includes a particle conveying body made up of a light-transmitting photoconductive layer, an insulative screen provided on the conductive layer and formed with a plurality of pores to be filled with the colored particles, and an electrode layer formed on the top of the screen. A particle feeding section feeds the colored particles charged to a first polarity to the particle conveying body. A facing electrode faces the particle conveying body with the intermediary of a recording medium. An exposing member exposes the colored particles via the conductive layer in accordance with an image signal to thereby charge the particles to a second polarity. A first electric field applying device applies a first electric field, which electrically attracts the colored particles charged to the first polarity toward the conductive layer, between the conductive layer and the electrode layer. A second electric field applying device applies a second electric field, which electrically attracts the charged particles charged to the second polarity toward the facing electrode, between the facing electrode and the conductive layer. A body driving device causes the particle conveying body to move between the particle feeding section and the facing electrode in circulation. Also, in accordance with the present invention, an image forming method begins with a step of uniformly charging photoconductive, colored particles to a first polarity. The colored particles charged to the first polarity are caused to fill a plurality of pores of a particle conveying body that is made up of a conductive layer transparent for light, an insulative screen provided on the conductive layer and formed with the pores, and an electrode layer formed on the top of the screen. Light for exposure is radiated from the bottom side of the pores. A first electric field, which electrically attracts the colored particles charged to the first polarity toward the conductive layer, is formed between the electrode layer and the conductive layer. The light and first electric field are caused to charge the colored particles to a second polarity opposite to the first polarity. A second electric field is formed between a facing electrode, which faces the particle conveying body with the intermediary of a recording medium, and the conductive layer to thereby cause the colored particles to fly toward and deposit on the recording medium.